Vehicle communication systems to lock and unlock a vehicle door without the use of a mechanical key have been in practical use. Specifically, such systems as follows have been in practical use: a keyless entry system to lock or unlock a vehicle door by a radio remote control operation using a mobile device carried by the user; and a smart entry (trademark) system to unlock a vehicle door by the user who are carrying the mobile device only approaching the vehicle or grasping a door handle.
Moreover, a vehicle communication system to start the engine of the vehicle without the use of a mechanical key has also been in practical use. Specifically, a push start system to start the engine by the user who are carrying the mobile device only pressing an engine start button has been in practical use.
In the above communication systems, while the vehicle-mounted device performs communication with the mobile device by a radio signal, performs authentication and then, performs control relates to a predetermined operation such as unlocking, locking or engine starting, in order to prevent an invalid operation, before performing the operations, whether the mobile device is present in a predetermined position or not is determined. As methods for checking the position of the mobile device, the methods described in Japanese Patent Application Laid-Open Publication No. 2009-144367, Japanese Patent Application Laid-Open Publication No. 2015-13527 and so on are known.
Japanese Patent Application Laid-Open Publication No. 2009-144367 discloses a position detection method in which a signal is transmitted from a transmitting antenna provided in the vehicle cabin and whether the mobile device is present inside the vehicle cabin or not is determined based on whether the mobile device responds to the signal or not. The signal transmitted from the transmitting antenna has a predetermined signal intensity, and the signal intensity is set to an intensity such that the mobile device can receive the signal when it is present inside the vehicle cabin and cannot receive the signal when it is present outside the vehicle cabin. Since the reachable range of signals having the same intensity spreads substantially in a spherical shape with the transmitting antenna as the center, it is desirable that the transmitting antenna be disposed substantially on the center line in the width direction of the vehicle. However, on the center line in the width direction of the vehicle, various devices such as a console and a parking brake are sometimes disposed, and there are cases where the transmitting antenna cannot be disposed on the center line in the width direction and is disposed on the right side or the left side in the width direction. When the transmitting antenna is disposed on the right side, if the signal intensity is set so that the signal reaches the right side surface of the vehicle, the signal does not reach the left side surface, so that there are cases where the mobile device present in the neighborhood of the left side surface inside the vehicle cabin is erroneously determined to be present outside the vehicle cabin. Moreover, if the signal intensity is set so that the signal reaches the left side surface of the vehicle, the signal reaches the outside of the right side surface of the vehicle cabin, so that there are cases where the mobile device present in the neighborhood of the right side surface outside the vehicle cabin is erroneously determined to be present inside the vehicle cabin. As a technology to solve this problem, Japanese Patent Application Laid-Open Publication No. 2009-144367 discloses a structure in which when a request switch on the right side surface of the vehicle is operated, switching is made to a signal intensity so that the signal reaches the right side surface of the vehicle and when a request switch on the left side surface is operated, switching is made to a signal intensity so that the signal reaches the left side surface of the vehicle.
Japanese Patent Application Laid-Open Publication No. 2015-13527 discloses a radio communication system in which a signal is transmitted from a transmitting antenna provided in the vehicle cabin to a mobile device, the mobile device is determined to be present outside the vehicle cabin when the received signal strength of the signal received by the mobile device is less than a threshold value and the mobile device is determined to be present inside the vehicle cabin when the intensity is not less than the threshold value.
In particular, Japanese Patent Application Laid-Open Publication No. 2015-13527 discloses a technology in which whether the mobile device is present in the neighborhood of a side surface of the vehicle or not is conjectured by using a transmitting antenna provided outside the vehicle cabin, the threshold value is set to a high value when the mobile device is present in the neighborhood of the side surface and the threshold value is set to a low value when the mobile device is not present in the neighborhood of the side surface.
On the other hand, as a general method for determining the position of a mobile device, Japanese Patent Application Laid-Open Publication No. 2013-100672 discloses a vehicle-mounted communication system in which the vehicle interior area is divided into a plurality of areas and the received signal strengths of a plurality of vehicle-mounted antennas received by the mobile device in each area are compared with a threshold value to determine whether the mobile device is present in the area.
Moreover, Japanese Patent Application Laid-Open Publication No. 2012-177242 discloses an electronic key system in which a plurality of LF oscillators that transmit signals of intensities that can reach a range containing the vehicle interior space are provided and when a response signal from the mobile device to the signals transmitted from the plurality of LF oscillators is present, the mobile device is determined to be present inside the vehicle cabin.